In one category of touch-sensitive panels known as ‘above surface optical touch systems’ and known from e.g. U.S. Pat. No. 4,459,476, a plurality of optical emitters and optical receivers are arranged around the periphery of a touch surface to create a grid of intersecting light paths above the touch surface. Each light path extends between a respective emitter/receiver pair. An object that touches the touch surface will block certain ones of the light paths. Based on the identity of the receivers detecting a blocked light path, a processor can determine the location of the intercept between the blocked light paths. This type of system is only capable of detecting the location of one object (single-touch detection). Further, the required number of emitters and receivers, and thus cost and complexity, increases rapidly with increasing surface area and/or spatial resolution of the touch panel.
In a variant, e.g. shown in WO2006/095320, each optical emitter emits a beam of light that diverges across the touch surface, and each beam is detected by more than one optical receiver positioned around the periphery of the touch surface. Thus, each emitter creates more than one light path across the touch surface. A large number of light paths are created by sequentially activating different emitters around the periphery of the touch surface, and detecting the light received from each emitter by a plurality of optical receivers. Thereby, it is possible to reduce the number of emitters and receivers for a given surface area or spatial resolution, or to enable simultaneous location detection of more than one touching object (multi-touch detection).
If the display screen is contaminated by e.g. fingerprints, the optical transmission path may become unintentionally interrupted and the information retrieved from the system erroneous or incomplete as the contaminated surface becomes insensitive to touches. If contaminants are collected in front of one of the emitters or detectors there will always be blocked or occluded light paths.
Another category of touch-sensitive panels known as ‘in-glass optical systems’ is now described and is also known from e.g. U.S. Pat. No. 8,581,884.
FIG. 1 illustrates an example of a touch-sensitive apparatus 100 that is based on the concept of FTIR (Frustrated Total Internal Reflection), also denoted “FTIR system”. The apparatus operates by transmitting light inside a transmissive panel 10, from light emitters 30a to light sensors or detectors 30b, so as to illuminate a touch surface 20 from within the transmissive panel 10. The transmissive panel 10 is made of solid material in one or more layers and may have any shape. The transmissive panel 10 defines an internal radiation propagation channel, in which light propagates by internal reflections.
In the example of FIG. 1, the propagation channel is defined between the touch surface 20 and bottom surface 25 of the transmissive panel 10, where the touch surface 20 allows the propagating light to interact with touching object 60 and thereby defines the touch surface 20. This is achieved by injecting the light into the transmissive panel 10 via coupling element 40 such that the light is reflected by total internal reflection (TIR) in the touch surface 20 as it propagates through the transmissive panel 10. The light may be reflected by TIR on the bottom surface 25 or against a reflective coating thereon. Upon reaching coupling element 40 on a far side of the panel, the light is coupled out of transmissive panel 10 and onto detectors 30b. The touch-sensitive apparatus 100 may be designed to be overlaid on or integrated into a display device or monitor.
U.S. Pat. No. 8,553,014 describes an attempt to combine the above surface and in-glass optical systems described above. U.S. Pat. No. 8,553,014 describes an optical coupling technique for introducing light into a transmissive panel and above a transmissive panel simultaneously. However, the in-coupling component shown in FIG. 126 of U.S. Pat. No. 8,553,014 is a complex prism and appears to rely on total internal reflection and diffraction to couple the light above the touch surface. Such an arrangement would be highly tolerance sensitive, making the optical signal highly sensitive to, for example, the load on the touch surface, the tolerances of process used to mount the prism to the transmissive panel, and the manufacturing of both the transmissive panel and the prism. Furthermore, the spread of the light in a plane parallel to the transmissive panel is limited to a range of less than 80 degrees as light outside this range will be diffracted up and away from the panel. Such a system is best suited to a rectangular grid of detection lines, such as described in U.S. Pat. No. 4,459,476 above. Furthermore, a complex prism as described in U.S. Pat. No. 8,553,014 would be both expensive to manufacture and bulky, taking up valuable space underneath and to the side of the transmissive panel.